183 



(Cashaqna) shales, at Section ] ; in the Genesee shale, at 

 Sections 1 and 2, and on the Lake Shore; in the Styliolina, 

 band, at Sections 1, 2, :>; in the Demissa bed, at Section T) ; 

 and between twelve and fourteen feet below the Encrinal 

 limestone, at Section 7. 



Measurements of a ver}- larov individual were G nun. 

 long- and 8.5 mm. wide. Usually they are much smallei*. 



Gexus ORBiriTLOIDEA. D^Orbionv. 



[Ety. : Orhicula, a g'eiuis; oide)<, like.] 



(1850: Prodr. de Palu(nit, T. I., p. 44. 1892: Pal. X. Y., 

 Vol. VIII., Pt. I., p. 120.) 



Shell varying from nearly circular to almost elliptical in 

 outline, the valves unequal; apices eccentric; lower (])edicle) 

 valve fiat, or nearly so; a narrow pedicle furrow passes 

 backward from just behind the beak, and ends near the 

 margin in a short tubular sipho, which peneti-ates the 

 shell and emerges on the inner sides, near the margin ; upper 

 (brachial) valve conical, apex directed backwards; surface 

 marked by concentric lines of growth; shell substance 

 partly corneous. 



Orbiculoidea MEni.\. Hall. (Fig. 82.) (Pal. N. Y., Vol. 

 IV., p. 20, PI. II. Disciim medin. ) 



Distinguishing Clniravtevs. — Size; broadly elliptical or 

 nearly circular outline; de])ressed bi-achial valve, with apex 

 at one-third the distance from the pos- 

 terior margin; pedicle furrow commonly 

 in the longitudinal axis; tine regular 

 surface stria?. p^^_ ^., ,,,„,,„,,,■,,,, 



Found in the upper Moscow shales ^^i^^; .Sal^^c^ml 

 (transition shales), at Section 3; in the "'*'''■ 

 middle Moscow (O. media) bed, at Sections 4 and 5; at ten 

 feet below, and between fourteen and seventeen feet below 

 the Encrinal limestone, at Section 7 ; and in the Modionior- 

 pha suhalata bed, on the Lake Shore. 



Orbiculoidea DORU. Hall. (Fig. 83.) (Pal. N. Y.. Vol. 

 IV., p. 19, PI. II.) 



